The present invention relates to light transmission tubes. Each of the light transmission tubes to which the present invention relates is comprised of a transparent tubular clad and a transparent core section having a higher refractive index than the transparent tubular clad. In particular, this invention relates to light transmission tubes each of which is capable of emitting a light having a predetermined directivity from one side of the clad (a part of its outer surface area).
In general, if a light transmission tube is comprised of a tubular clad and a core section having a higher refractive index than the tubular clad, a usually adopted method for the use of it is that a large amount of light be sent toward the front end of the light transmission tube. As a result, a brightness on the circumferential surface area of the light transmission tube is usually not very high. As a method for increasing the above brightness, it is allowable for people to consider forming some irregular convex and concave portions on the internal surface of the clad so as to improve the brightness on the circumferential surface area of the light transmission tube. However, if a light transmission tube has a structure which is formed by filling the internal space of the tubular clad with a liquid state polymerizable monomer (for forming the core section), followed by applying a pressure to effect a predetermined polymerization of the monomer, the formation of the irregular convex and concave portions on the internal surface of the clad will cause the clad to be easily broken, hence making it difficult to manufacture a light transmission tube in the above manner.
Further, it has been also suggested that a sort of particles having a desired light scatterring property be dispersed throughout the core section so as to increase the brightness on the circumferential surface area of the light transmission tube. However, although it has been suggested that the particles capable of light scatterring be at first dispersed in a liquid polymerizable monomer, then followed by a predetermined polymerization and solidification, there has not been suggested a method in which an amount of light scatterring particles be added in the liquid state monomer so that upon polymerization of the monomer the light scatterring particles may be dispersed or a light reflective layer may be formed in a desired manner.
In the past, as a light emitting tubular member capable of emitting a light along a length of about several meters, there had been in use a neon tube and a fluorescent tube. However, when using a neon tube or a fluorescent tube, since a high voltage is needed to be applied to a neon tube or a fluorescent tube, there is a danger that an electric shock or an electric leakage may happen. As a result, such a neon tube or a fluorescent tube can not be used under water, nor is it suitable for them to be used in a place where rain water or snow frequently enters. Further, since a neon tube or a fluorescent tube has been formed into a tubular shape, they do not have a desired impact resistance. For this reason, both a neon tube and a fluorescent tube failed to be used in a place where other objects such as people and vehicles might appear.
In order to solve the above problem, it has been suggested to use a light transmission tube which is a flexible tube filled with a transparent core liquid or a soft transparent polymer, or to employ a light transmission tube involving the use of a plastic material and an optical fiber. Each of the above suggested light transmission tubes is so formed that a light from a light source is allowed to be introduced into the tube through one end thereof and then this light is emitted from the circumferential surface of the tube along a length that may be several tens of meters. In this way, since a light source may be separated from a light emitting portion, the tube would have no danger of being broken. As a result, each of the above suggested light transmission tubes is suitable for use under water or in an outdoor environment or even in an environment which might involve a possibility of an explosion. Further, since each of the above suggested light transmission tubes is allowed to be manufactured without having to perform some troublesome treatment such as glass delicate treatment, a corresponding manufacturing process may be carried out with an improved efficiency.
On the other hand, with a light transmission tube which is comprised of a transparent tubular clad and a transparent core section having a higher refractive index than the transparent tubular clad, since it is usually preferred that a lot of light be sent toward the front end of the light transmission tube, a brightness on the circumferential surface area of the light transmission tube is usually not very high.
In order to increase the above brightness, it is allowable to consider the formation of some irregular convex and concave portions on the internal surface of the clad so as to improve the brightness on the circumferential surface area of the light transmission tube. However, since in use of such a light transmission tube, a light will be caused to emit from the circumferential surface area of the tube in every direction along the cross sectional plane, it is impossible to obtain a light emission having a sufficiently high brightness in a predetermined desired direction.
To cope with the above problem, it has been further suggested that a light reflective coating material is applied in a dotted or linear pattern to the outer circumferential surface of a rod member which is made of glass or a transparent resin, so as to form a light reflective layer thereon, thereby providing a light directing rod enabling a light to be emitted in a specifically determined direction. With the use of such a light directing rod, since a light is caused to be emitted in a certain specifically determined direction, a desired brightness of the light emission may be increased correspondingly. But, a problem with such a light directing rod is that if some dust or small rubbish is attached to the outer circumferential surface of the light directing rod, such dust or small rubbish will cause a deterioration in the light transmissibility of the transparent rod. As a result, a desired brightness will be decreased and/or a brightness distribution in the longitudinal direction of the transparent rod will have an undesired variation. Because of this, such a light directing transparent rod can only be used in a very clean environment free of dust or small rubbish. Moreover, the above light directing rod is manufactured by using an extrusion molding method or an injection molding method so as to produce a transparent rod, followed by printing a light reflective coating material on to the outer circumferential surface of the transparent rod. In fact, since a relatively long time is required in a subsequent drying process for drying the printed coating material, and since the whole manufacturing process has to involve at least two steps including rod formation and coating material printing, an industrial productivity is low and its manufacturing cost is high. In addition, since the above printing process is performed on a rod-like member, a desired reflective layer can only be formed on one side of the outer circumferential surface thereof, hence undesirably causing an emitted light to receive a restriction in its emitting direction.
As may be understood from the above discussion, in any of the related prior art techniques, there has not been provided a light transmission tube whose brightness may be increased by emitting a light from a circumferential surface of said tube in a specifically determined direction, which light transmission tube is capable of being used without a problem that its light emitting performance will be possibly deteriorated because of an attachment of dust or small rubbish thereto. Further, it is desired that a light transmission tube can be produced in a large amount on an industrial level with only a low cost.